1Heat the oven to 400°F and arrange a rack in the middle. Lightly dust a baking sheet with all-purpose flour; set aside.
2Place both flours, baking soda, and salt in a large bowl and whisk to combine, breaking up any lumps. Add buttermilk and melted butter and mix with your hands until almost all of the flour is moistened and the dough holds together, about 1 minute.
3Lightly flour a clean work surface and turn out the dough. Knead until it forms a fairly smooth ball with no visible pockets of flour, about 1 minute. Work the dough into a flat round about 7 inches in diameter and 2 inches thick. Place on the prepared baking sheet and, using a sharp knife, slice an “X” across the top, edge to edge and about 1/2 inch deep.
4Bake until the internal temperature registers 190°F to 200°F on an instant-read thermometer and the bread makes a hollow sound when tapped, about 35 to 40 minutes. Transfer to a wire rack and cool completely before slicing, about 2 hours. (If you slice the bread before it has completely cooled, it will be crumbly or fall apart.)